Explicitly use language_c when evaluating a SDT probe argument

Explicitly use language_c when evaluating a SDT probe argument

Joel contacted me offlist with a question about a warning that one of
his customers was seeing.  The message came from the new
linker-debugger interface, which uses SDT probes internally.  The
warning said:

    (gdb) run
    [...]
    warning: Probes-based dynamic linker interface failed.
    Reverting to original interface.

    Argument to arithmetic operation not a number or boolean.

This should not have happened in the environment the customer was
using (RHEL-6.x), so I found it strange.  Another thing caught my
attention: the last message, saying "Argument to arithmetic operation
not a number or boolean.".

Joel kindly investigated the issue further, and found the answer for
this.  To quote him:

	(gdb) set lang c
	(gdb) p 48+$ebp
	$4 = (void *) 0xffffd0f8

    So far so good. But...

	(gdb) set lang ada
	(gdb) p 48+$ebp
	Argument to arithmetic operation not a number or boolean.

    Ooops! Interestingly, if you revert the order of the operands...

	(gdb) p $ebp+48
	$5 = (access void) 0xffffd0f8

So the problem is doing pointer arithmetics when the language is set
to Ada.

I remembered that, during the parsing and the evaluation of SDT probe
arguments, the code sets the language as current_language, because, at
that time, I thought it was not necessary to worry about the language
given that the code implements its own parser.  I was wrong.  So here
is a patch to fix that, by setting the language as C, which should
guarantee that the maths are done in the right way (TM).

It was somewhat hard to find a reproducer for this issue.  In the end,
what I had to do was to create a testcase that used the %ebp register
on some displacement (e.g., "-4(%ebp)"), which finally triggered the
bug.  I am not sure why I could not trigger it when using other
registers, but I did not want to spend too much time investigating
this issue, which seemed like an Ada issue.  Also, because of this
peculiar way to trigger the problem, the testcase only covers x86-like
targets (i.e., i*86 and x86_64 with -m32).

Joel kindly tested this for me, and it worked.  I also ran a full
regression test here on my Fedora 20 x86_64, and everything is fine.

I will push this patch in a few days if there are no comments.

gdb/ChangeLog:
2014-10-14  Sergio Durigan Junior  <sergiodj@redhat.com>

	* stap-probe.c (stap_parse_argument): Initialize expout explicitly
	using language_c, instead of current_language.

gdb/testsuite/ChangeLog:
2014-10-14  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.arch/stap-eval-lang-ada.S: Likewise.
	* gdb.arch/stap-eval-lang-ada.c: Likewise.
	* gdb.arch/stap-eval-lang-ada.exp: New file.